The high affinity interaction between the HIV-1-envelope (gp120/41) and its receptor CD4, was the basis for several clinical protocols in which sCD4 or chimeric CD4-Ig(Fc) were used in treatment of HIV-1 seropositive patients. These trials did not result in success thus far. Our goals were : (a) To study the kinetics of the interactions between sCD4 and HIV-1 env-expressing cells, in relation to subsequent events leading to cell fusion and inhibition of syncytia formation. Methods: The association and dissociation rates of sCD4 interactions with env-expressing cells, and the quantification of sCD4-induced gp120 shedding was determined by a quantitative flow cytometry assay. Syncytia inhibition was measured in the continuous presence of sCD4, or after washing of cells following pre-incubation with sCD4. Results: The kinetics of syncytia formation inhibition correlated with sCD4 binding when sCD4 was maintained during the culture. However, when the env-expressing cells were preincubated with sCD4 and then washed to remove unbound sCD4, no syncytia inhibition was observed, even following sCD4-induced shedding of >50% of surface gp120 molecules. Conclusions: The lack of syncytia inhibition seen after removal of unbound sCD4, even after pre-incubation of cells under saturation and gp120-shedding conditions, were explained by the rapid dissociation of the bound sCD4 from the env-expressing cells (k/min: 1.9 x 10e-2), allowing cell fusion to continue. It also suggested that sufficient numbers of fusogenic molecules remained on the sCD4-treated cells. These studies are important for understanding HIV-1env-mediated cell fusion and AIDS therapy with CD4 derivatives, or other agents (e.g. anti-HIV-1env monoclonal antibodies).